A magnetic disk drive comprises a disk (more specifically a magnetic disk) as a recording medium. Now, it is assumed that a very small protrusion is present on a surface of the disk. In this case, a head (more specifically a magnetic head) may fail to write data to an area on the disk where the protrusion is present or to read data from the area. Furthermore, the head may collide against the protrusion and be damaged. To avoid such an event, various methods for detecting defects such as protrusions on a disk have been proposed.
However, defects detected according to the conventional technique are not limited to protrusions. That is, according to the conventional technique, a recess in the surface of the disk is also detected as a defect. Thus, the conventional technique detects both protrusions (that is, protruding defects) and recesses (that is, recessed defects) as defects without distinguishing between the recesses and the protrusions. However, the protruding defect and the recessed defect have different degrees of adverse effects in connection with the above-described event. As a result, there has been a demand for a technique for determining whether a detected defect is a recess or a protrusion.